Continuous plain waterjets (CWJ) have been used in the prior art to prep metallic and non-metallic surfaces. Continuous plain waterjets are waterjets that are not modulated or pulsed. To prep surfaces using these conventional continuous plain waterjets, these waterjets must typically be operated at very high pressures such as, for example, pressures of approximately 60,000 psi. Operating continuous plain waterjets at such high pressures not only requires expensive high-pressure pumps, lines, fittings, etc., but also utilizes copious amounts of energy. These very high pressure waterjets are thus expensive and prone to breakdown.
Examples of continuous-flow, high-pressure waterjet systems for cutting and cleaning are disclosed in U.S. Pat. No. 4,787,178 (Morgan et al.), U.S. Pat. No. 4,966,059 (Landeck), U.S. Pat. No. 6,533,640 (Nopwaskey et al.), U.S. Pat. No. 5,584,016 (Varghese et al.), U.S. Pat. No. 5,778,713 (Butler et al.), U.S. Pat. No. 6,021,699 (Caspar), U.S. Pat. No. 6,126,524 (Shepherd) and U.S. Pat. No. 6,220,529 (Xu). Further examples are found in European Patent Applications EP 0 810 038 (Munoz) and EP 0 983 827 (Zumstein), as well as in US Patent Application Publications 2002/0109017 (Rogers et al.), 2002/0124868 (Rice et al.), and 2002/0173220 (Lewin et al.).
As noted above, continuous-flow waterjet technology, of which the foregoing are examples, suffers from certain drawbacks which render continuous-flow waterjet systems expensive and cumbersome. As persons skilled in the art have come to appreciate, continuous-flow waterjet equipment must be robustly designed to withstand the extremely high water pressures involved. Consequently, the nozzle, water lines and fittings are bulky, heavy and expensive. To deliver an ultra-high-pressure waterjet, an expensive ultra-high-pressure water pump is required, which further increases costs both in terms of the capital cost of such a pump and the energy costs associated with running such a pump.
In response to the shortcomings of continuous-flow waterjets, an ultrasonically pulsating nozzle was developed to deliver high-frequency modulated water in non-continuous, discrete packets, or “slugs”. This ultrasonic nozzle is described and illustrated in detail in U.S. Pat. No. 5,134,347 (Vijay) which issued on Oct. 13, 1992. The ultrasonic nozzle disclosed in U.S. Pat. No. 5,134,347 transduced ultrasonic oscillations from an ultrasonic generator into ultra-high frequency mechanical vibrations capable of imparting thousands of pulses per second to the waterjet as it travels through the nozzle. The waterjet pulses impart a waterhammer pressure onto the surface to be cut or cleaned. Because of this rapid bombardment of mini-slugs of water, each imparting a waterhammer pressure on the target surface, the erosive capacity of the waterjet is tremendously enhanced. The ultrasonically pulsating nozzle is thus able to cut or clean much more efficiently than the prior-art continuous-flow waterjets.
Theoretically, the erosive pressure of a continuous waterjet striking the target surface is the stagnation pressure, or ½ρv2 (where ρ represents the water density and v represents the impact velocity of the water as it impinges on the target surface). The pressure arising due to the waterhammer phenomenon, by contrast, is ρcv (where c represents the speed of sound in water, which is approximately 1524 m/s).
Thus, the theoretical magnification of impact pressure achieved by pulsating waterjet is 2c/v. As an example, if the impact velocity is 1,200 ft/s (372 m/s), generated by a pump operating at 10 kpsi (69 MPa), the magnification would be eight. Even if air drag neglected and the impact velocity is assumed to approximate the fluid discharge velocity of 1500 feet per second (or approximately 465 m/s), the magnification of impact pressure is about 6 to 7. If the model takes into account air drag, and assuming an impact velocity of about 300 m/s, then the theoretical magnification would be tenfold.
In practice, due to aerodynamic drag on the pulses and due to frictional and other inefficiencies, the pulsating ultrasonic nozzle described in U.S. Pat. No. 5,154,347 imparts about 3 to 5 times more impact pressure onto the target surface for a given source pressure. Therefore, to achieve the same erosive capacity, the pulsating nozzle need only operate with a pressure source that is 3 to 5 times less powerful. Since the pulsating nozzle may be used with a much smaller and less expensive pump, it is more economical than continuous-flow waterjet nozzles. Further, since waterjet pressure in the nozzle, lines, and fittings is much less with an ultrasonic nozzle, the ultrasonic nozzle can be designed to be lighter, less cumbersome and more cost-effective.
Although the basic ultrasonic nozzle described in U.S. Pat. No. 5,154,347 and the improvements presented in WO/2005/042177) entitled ULTRASONIC WATERJET APPARATUS (which are both hereby incorporated by reference) represent substantial breakthroughs in waterjet technology, in these early technologies only cursory/scant attention was paid to surface prepping. Accordingly, a method and apparatus for prepping surfaces that improves on the prior art technology would be highly desirable. These innovations and improvements are disclosed by Applicants in the present application.